Cyclin E is a critical cell cycle regulatory protein. At the same time, deregulation of cyclin E expression has been linked to carcinogenesis both in human patients and in mouse models. The current proposal constitutes an ongoing project aimed at gaining understanding into how cyclin E is normally regulated and to elucidate the mechanisms whereby cyclin E expression becomes deregulated and promotes carcinogenesis, respectively. The proposal is divided into three specific aims, the first of which addresses the mechanism of cyclin E degradation in cultured cells and in mice. The role of specific phosphorylation sites on cyclin E will be explored as well as the contribution of a new SCF protein ubiquitin ligase defined by the F-box protein hCdc4. The second specific aim is targeted at the mechanism whereby deregulation of cyclin E confers genomic instability, likely to be a contributing factor in cyclin E-mediated carcinogenesis. Two hypotheses will be explored in detail: that deregulation of cyclin E impairs DNA replication by interfering with pre-replication complex assembly and that elevated cyclin E levels in mitosis block the metaphase-anaphase transition by inhibiting the essential mitotic protein ubiquitin ligase known as APC. The final specific aim seeks to gain a better understanding of the link between cyclin E deregulation and carcinogenesis. Mouse models will be employed to determine if cyclin E promotes carcinogenesis by accelerating loss of heterozygosity (LOH) at tumor suppressor loci. It is hoped that these investigations will provide insights that will ultimately lead to improved prognosis and therapy.